# Chemical Kinetics - change to system conditions

This is a question from a very old AP Chem exam (2006) prep book which I am amusing myself with in my spare time which is kind of humbling. There is something I am missing with this question, one of several similar questions in the kinetics section.

Given the following reaction (which doesn't establish an equilibrium) :

$$\ce{A (g) + B (g) -> C (g)}$$

which of the following would increase the concentration of $$\ce{A}$$?

• addition of $$\ce{A}$$ at constant $$T$$ and $$V$$
• addition of $$\ce{A}$$ at constant $$T$$ and $$P$$
• addition of $$\ce{A}$$ at constant $$V$$ and $$P$$

Now my first thoughts are to reason as follows :

If you add $$\ce{A}$$ and the volume is constant obviously the concentration of $$\ce{A}$$ increases. If you add $$\ce{A}$$ and keep $$T$$ and $$P$$ constant the $$V$$ will just expand and the concentration will be the same. But if I ignore the reaction this is a gas law / concentration question so it is out of place. But it isn't an equilibrium so none of that reasoning applies either.

Now if the reaction happens so slow it takes a year for $$1\,\%$$ of $$\ce{A}$$ to react I could just consider the reaction not even happening? If the reaction happens so fast that it is essentially over in 1 second then it seems like any change to A would be undone as fast as you could increase it. But it seems like I am missing something basic as there is nothing given regarding the rate.

There are several questions almost identical to this in the same section which just have other than $$1:1$$ coefficients in the equation.

• It's not really kinetics, but concentration question, it seems. – Mithoron Dec 29 '16 at 15:53